Inductive monitoring of conductive trench depth

ABSTRACT

In fabrication of an integrated circuit having a layer with a plurality of conductive interconnects, a layer of a substrate is polished to provide the layer of the integrated circuit. The layer of the substrate includes conductive lines to provide the conductive interconnects. The layer of the substrate includes a closed conductive loop formed of a conductive material in a trench. A depth of the conductive material in the trench is monitored using an inductive monitoring system and a signal is generated. Monitoring includes generating a magnetic field that intermittently passes through the closed conductive loop. A sequence of values over time is extracted from the signal, the sequence of values representing the depth of the conductive material over time.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of and claims priority toU.S. application Ser. No. 14/312,503, filed on Jun. 23, 2014, which isincorporated by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to inductive monitoring during chemicalmechanical polishing of substrates.

BACKGROUND

An integrated circuit is typically formed on a substrate by thesequential deposition of conductive, semiconductive, or insulativelayers on a silicon wafer. A variety of fabrication processes requireplanarization of a layer on the substrate. For example, one fabricationstep involves depositing a conductive filler layer on a patternedinsulative layer to fill the trenches or holes in the insulative layer.The filler layer is then polished until the raised pattern of theinsulative layer is exposed. After planarization, the portions of theconductive filler layer remaining between the raised pattern of theinsulative layer form vias, plugs and lines that provide conductivepaths between thin film circuits on the substrate.

Chemical mechanical polishing (CMP) is one accepted method ofplanarization. This planarization method typically requires that thesubstrate be mounted on a carrier head. The exposed surface of thesubstrate is placed against a rotating polishing pad. The carrier headprovides a controllable load on the substrate to push it against thepolishing pad. A polishing liquid, such as slurry with abrasiveparticles, is supplied to the surface of the polishing pad.

One problem in CMP is determining whether the polishing process iscomplete, i.e., whether a substrate layer has been planarized to adesired flatness or thickness, or when a desired amount of material hasbeen removed. Variations in the slurry composition, the polishing padcondition, the relative speed between the polishing pad and thesubstrate, the initial thickness of the substrate layer, and the load onthe substrate can cause variations in the material removal rate. Thesevariations cause variations in the time needed to reach the polishingendpoint. Therefore, determining the polishing endpoint merely as afunction of polishing time can lead to non-uniformity within a wafer orfrom wafer to wafer.

In some systems, a substrate is monitored in-situ during polishing,e.g., through the polishing pad. One monitoring technique is to inducean eddy current in the conductive layer and detect the change in theeddy current as the conductive layer is removed.

SUMMARY

In some integrated circuit fabrication processes, polishing continuesafter the patterned insulative layer has been exposed, e.g., in order toreduce the depth of the conductive lines in the trenches. It would bedesirable to reliably halt polishing of the substrate when the trencheshave a target depth. However, due to the small line width of thetrenches, it can be difficult to induce eddy currents in the conductivelines. Consequently, conventional eddy current monitoring techniques maynot be sufficient to reliably determine the depth of the trenches, andthus may not reliably halt polishing when the trenches have the targetdepth.

However, an alternative approach is to incorporate a conductive loopinto the substrate being polished. Passage of a magnetic field throughthe conductive loop can induce a current in the loop. With respect tothe voltage source that generates the magnetic field, the conductiveloop generally acts as an impedance that depends on the depth of theconductive material. This permits generation of a signal that depends onthe depth of the conductive material in the trench.

In one aspect, a method of chemical mechanical polishing a substrateincludes, in fabrication of an integrated circuit having a layer with aplurality of conductive interconnects, polishing a layer of a substrateto provide the layer of the integrated circuit, wherein the layer of thesubstrate includes conductive lines to provide the conductiveinterconnects. The layer of the substrate includes a closed conductiveloop formed of a conductive material in a trench. A depth of theconductive material in the trench is monitored using an inductivemonitoring system and a signal is generated. Monitoring includesgenerating a magnetic field that intermittently passes through theclosed conductive loop. A sequence of values over time is extracted fromthe signal, the sequence of values representing the depth of theconductive material over time. A polishing endpoint is detected bydetermining from the sequence of values that a depth of the conductivematerial has reached a target depth, or at least one pressure applied bya carrier head to the substrate during polishing of the layer isadjusted based on the sequence of values such that different zones onthe substrate have closer to the same endpoint time than without such anadjustment.

In another aspect, a method of chemical mechanical polishing a substrateincludes, in fabrication of an integrated circuit having a layer with aplurality of conductive interconnects, polishing a layer of a substrateto provide the layer of the integrated circuit. The layer of thesubstrate includes conductive lines to provide the conductiveinterconnects, and the layer of the substrate includes a closedconductive loop formed of a conductive material in a trench. A depth ofthe conductive material in the trench is monitored using an inductivemonitoring system and a signal is generated. Monitoring includesgenerating a magnetic field from a core having a prong orientedsubstantially perpendicular to the layer of the substrate. The magneticfield intermittently passes through the closed conductive loop. Alateral dimension of the closed conductive loop is about 1-2 times alateral dimension of the prong.

In another aspect, a computer program product or a polishing system isprovided that carries out these methods.

In another aspect, a substrate for use in fabrication of an integratedcircuit has a layer with a plurality of conductive interconnects. Thesubstrate includes a semiconductor body, a dielectric layer disposedover the semiconductor body, a plurality of conductive lines of aconductive material disposed in first trenches in the dielectric layerto provide the conductive interconnects, and a closed conductive loopstructure of the conductive material disposed in second trenches in thedielectric layer. The closed conductive loop structure includes aplurality of openings through a conductive region to provide a pluralityof electrically connected conductive loops. The closed conductive loopis not electrically connected to any of the conductive lines.

In another aspect, a substrate for use in fabrication of an integratedcircuit has a layer with a plurality of conductive interconnects. Thesubstrate includes a semiconductor body, a first dielectric layerdisposed over the semiconductor body, a first plurality of conductivelines of a conductive material disposed in first trenches in the firstdielectric layer to provide at least some of the conductiveinterconnects, a first closed conductive loop structure of theconductive material disposed in second trenches in the first dielectriclayer, a second dielectric layer disposed over the first dielectriclayer, a second plurality of conductive lines of the conductive materialdisposed in third trenches in the second dielectric layer to provide atleast some of the conductive interconnects, and a second closedconductive loop structure of the conductive material disposed in fourthtrenches in the second dielectric layer, wherein a width of the secondclosed conductive loop structure is greater than a width of the firstclosed conductive loop structure.

Certain implementations can include one or more of the followingadvantages. The depth (or conductivity) of a conductive material, e.g.,a metal such as copper, in a trench, can be sensed. Polishing can behalted more reliably when the trenches have a target depth, and closedloop control of carrier head pressure can be performed to drive touniform metal line thickness and conductivity. Thus, the overallfabrication process can have improved yield.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other aspects, featuresand advantages will be apparent from the description and drawings, andfrom the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic side view, partially cross-sectional, of achemical mechanical polishing station that includes an inductivemonitoring system.

FIG. 2 is a schematic circuit diagram of portions of the inductivemonitoring system.

FIG. 3 is a schematic top view of a platen of a chemical mechanicalpolishing station.

FIG. 4A is a schematic top view of a substrate.

FIG. 4B is a schematic perspective view of a conductive loop on asubstrate.

FIG. 5 is a schematic cross-sectional view of a substrate, e.g., alongline 5 from FIG. 4A.

FIG. 6 is a schematic cross-sectional view of a substrate havingmultiple layers.

FIG. 7 is a schematic top view of a multiple conductive loop structure.

FIG. 8 illustrates a signal from an inductive monitoring system.

FIG. 9 illustrates a sequence of values generated by the inductivemonitoring system.

FIG. 10 illustrates two sequence of values generated by the inductivemonitoring system for two zones on a substrate.

FIGS. 11A-11E schematically illustrate polishing of a substrate.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

CMP systems can use an inductive monitoring system to detect the depthof a conductive material in a trench on a substrate. The measurementscan be used to halt polishing when the trenches have a target depth, orto adjust processing parameters of the polishing process in real time.For example, a substrate carrier head can adjust the pressure on thebackside of the so that the trenches in different regions of thesubstrate have substantially the same depth after polishing.

FIG. 1 illustrates an example of a polishing station 20 of a chemicalmechanical polishing apparatus. The polishing station 20 includes arotatable disk-shaped platen 24 on which a polishing pad 30 is situated.The platen 24 is operable to rotate about an axis 25. For example, amotor 22 can turn a drive shaft 28 to rotate the platen 24. Thepolishing pad 30 can be a two-layer polishing pad with an outer layer 34and a softer backing layer 32.

The polishing station 20 can include a supply port or a combinedsupply-rinse arm 39 to dispense a polishing liquid 38, such as slurry,onto the polishing pad 30. The polishing station 20 can include a padconditioner apparatus with a conditioning disk to maintain the conditionof the polishing pad.

The carrier head 70 is operable to hold a substrate 100 against thepolishing pad 30. The carrier head 70 is suspended from a supportstructure 72, e.g., a carousel or a track, and is connected by a driveshaft 74 to a carrier head rotation motor 76 so that the carrier headcan rotate about an axis 71. Optionally, the carrier head 70 canoscillate laterally, e.g., on sliders on the carousel or track 72; or byrotational oscillation of the carousel itself. In operation, the platenis rotated about its central axis 25, and the carrier head is rotatedabout its central axis 71 and translated laterally across the topsurface of the polishing pad 30. Where there are multiple carrier heads,each carrier head 70 can have independent control of its polishingparameters, for example each carrier head can independently control thepressure applied to each respective substrate.

The carrier head 70 can include a flexible membrane 80 having asubstrate mounting surface to contact the back side of the substrate100, and a plurality of pressurizable chambers 82 to apply differentpressures to different zones, e.g., different radial zones, on thesubstrate 100. The carrier head can also include a retaining ring 84 tohold the substrate. In some implementations, the retaining ring 84 mayinclude a highly conductive portion, e.g., the carrier ring can includea thin lower plastic portion 86 that contacts the polishing pad, and athick upper conductive portion 88. In some implementations, the highlyconductive portion is a metal, e.g., the same metal as the layer beingpolished, e.g., copper or cobalt.

A recess 26 is formed in the platen 24, and optionally a thin section 36can be formed in the polishing pad 30 overlying the recess 26. Therecess 26 and thin pad section 36 can be positioned such that regardlessof the translational position of the carrier head they pass beneathsubstrate 100 during a portion of the platen rotation. Assuming that thepolishing pad 30 is a two-layer pad, the thin pad section 36 can beconstructed by removing a portion of the backing layer 32. The thinsection can optionally be optically transmissive, e.g., if an in-situoptical monitoring system is integrated into the platen 24.

An in-situ monitoring system 40 generates a time-varying sequence ofvalues that depend on the thickness of a conductive trench on thesubstrate 100. In particular, the in-situ monitoring system 40 can be aninductive monitoring system. In operation, the polishing station 22 usesthe monitoring system 40 to determine when the trenches have beenpolished to a target depth.

The inductive monitoring system 40 can include an inductive sensor 42installed in the recess 26 in the platen. The sensor 42 can include amagnetic core 44 positioned at least partially in the recess 26, and atleast one coil 46 wound around the core 44. Drive and sense circuitry 48is electrically connected to the coil 46. The drive and sense circuitry48 generates a signal that can be sent to a controller 90. Althoughillustrated as outside the platen 24, some or all of the drive and sensecircuitry 48 can be installed in the platen 24. A rotary coupler 29 canbe used to electrically connect components in the rotatable platen,e.g., the coil 46, to components outside the platen, e.g., the drive andsense circuitry 48.

The core 44 can include two (see FIG. 1) or three (see FIG. 2) prongs 50extending in parallel from a back portion 52. Implementations with onlyone prong (and no back portion) are also possible.

Referring to FIG. 2, the circuitry 48 applies an AC current to the coil46, which generates a magnetic field 50 between two poles 52 a and 52 bof the core 44. In operation, a portion of the magnetic field 50 extendsinto the substrate 100.

FIG. 2 illustrates an example of the drive and sense circuitry 48. Thecircuitry 48 includes a capacitor 60 connected in parallel with the coil46. Together the coil 46 and the capacitor 60 can form an LC resonanttank. In operation, a current generator 62 (e.g., a current generatorbased on a marginal oscillator circuit) drives the system at theresonant frequency of the LC tank circuit formed by the coil 46 (withinductance L) and the capacitor 60 (with capacitance C). The currentgenerator 62 can be designed to maintain the peak to peak amplitude ofthe sinusoidal oscillation at a constant value. A time-dependent voltagewith amplitude V₀ is rectified using a rectifier 64 and provided to afeedback circuit 66. The feedback circuit 66 determines a drive currentfor current generator 62 to keep the amplitude of the voltage V₀constant. Marginal oscillator circuits and feedback circuits are furtherdescribed in U.S. Pat. Nos. 4,000,458, and 7,112,960 which areincorporated by reference.

When the magnetic field 50 passes through a conductive loop on thesubstrate, the magnetic field 50 generates a current in the loop. Thisincreases the effective impedance, thus increasing the drive currentrequired for the current generator 62 to keep the amplitude of thevoltage V₀ constant. The extent of the increase of the effectiveimpedance depends on the conductivity of the loop, which depends on thedepth of the conductive material in the trench defining the loop. Inshort, power dissipation by the conductive loop linearly relates to thedepth of conductive material in the trench. Thus, the drive currentgenerated by the current generator 62 provides a measurement of thedepth of the conductive material in the trench.

Other configurations are possible for the drive and sense circuitry 48.For example, separate drive and sense coils could be wound around thecore, the drive coil could be driven at a constant frequency, and theamplitude or phase (relative to the driving oscillator) of the currentfrom the sense coil could be used for the signal.

Returning to FIG. 1, in some implementations, the polishing station 20includes a temperature sensor 92 to monitor a temperature in thepolishing station or a component of/in the polishing station. Althoughillustrated in FIG. 1 as positioned to monitor the temperature of thepolishing pad 30 and/or slurry 38 on the pad 30, the temperature sensor92 could be positioned inside the carrier head to measure thetemperature of the substrate 100. The temperature sensor can be indirect contact (i.e., a contacting sensor) with the polishing pad or theexposed surface of the substrate 100, or the temperature sensor can be anon-contacting sensor (e.g., an infrared sensor). The monitoredtemperature(s) can be used in adjusting the measurements from theinductive monitoring system.

In some implementations, a polishing apparatus includes additionalpolishing stations. For example, a polishing apparatus can include twoor three polishing stations. For example, the polishing apparatus caninclude a first polishing station with an eddy current monitoring systemand a second polishing station with an inductive monitoring system.

For example, in operation, bulk polishing of the conductive layer on thesubstrate can be performed at the first polishing station, and polishingcan be halted when the barrier layer or patterned dielectric layer isexposed. The substrate is then transferred to the second polishingstation, and the substrate can be polished until the trenches reach thetarget depth.

FIG. 3 illustrates a top view of the platen 24. As the platen 24rotates, the sensor 42 sweeps below the substrate 100. By sampling thesignal from the circuitry 48 at a particular frequency, the circuitry 48generates measurements at a sequence of sampling zones 94 across thesubstrate 100. For each sweep, measurements at one or more of thesampling zones 94 can be selected or combined. Thus, over multiplesweeps, the selected or combined measurements provide the time-varyingsequence of values. In addition, off-wafer measurements may be performedat the locations where the sensor 49 is not positioned under thesubstrate 100.

The polishing station 20 can also include a position sensor 96, such asan optical interrupter, to sense when the inductive sensor 42 isunderneath the substrate 100 and when the eddy current sensor 42 is offthe substrate. For example, the position sensor 96 can be mounted at afixed location opposite the carrier head 70. A flag 98 can be attachedto the periphery of the platen 24. The point of attachment and length ofthe flag 98 is selected so that it can signal the position sensor 96when the sensor 42 sweeps underneath the substrate 100.

Alternately, the polishing station 20 can include an encoder todetermine the angular position of the platen 24. The inductive sensorcan sweep underneath the substrate with each rotation of the platen.

A controller 90, e.g., a general purpose programmable digital computer,receives the sequence of values from the inductive monitoring system.Since the sensor 42 sweeps beneath the substrate with each rotation ofthe platen, information on the depth of the trenches is accumulatedin-situ and on a continuous real-time basis (once per platen rotation).The controller 90 can be programmed to sample measurements from themonitoring system when the substrate generally overlies the thin section36 (as determined by the position sensor). As polishing progresses, thethickness of the conductive layer changes, and the sampled signals varywith time. The measurements from the monitoring system can be displayedon an output device during polishing to permit the operator of thedevice to visually monitor the progress of the polishing operation.

In addition, the controller 90 can be programmed to divide themeasurements from both the inductive current monitoring system 40 fromeach sweep beneath the substrate into a plurality of sampling zones, tocalculate the radial position of each sampling zone, and to sort themeasurements into radial ranges.

The controller 90 may also be connected to the pressure mechanisms thatcontrol the pressure applied by carrier head 70, to carrier headrotation motor 76 to control the carrier head rotation rate, to theplaten rotation motor 22 to control the platen rotation rate, or toslurry distribution system 39 to control the slurry composition suppliedto the polishing pad. Specifically, after sorting the measurements intoradial ranges, information on the trench depth can be fed in real-timeinto a closed-loop controller to periodically or continuously modify thepolishing pressure profile applied by a carrier head, as discussedfurther below.

FIGS. 4A and 4B illustrate a substrate 100 that has a closed conductiveloop 102. In general, the substrate will have multiple closed conductiveloops 102, and the closed conductive loops can be distributed uniformlyacross the substrate. Each conductive loop 102 need not be connected toother interconnect wiring in the substrate; it can be a free-standingfeature on the substrate. The conductive loop can have a line width W(see FIG. 5) of about 0.5 to 10 um, depending on the metal layer beingused. The conductive loop 102 has the same depth as the otherinterconnect wiring in the layer.

In some implementations, the closed conductive loop 102 encircles a die104. For example, the closed conductive loop can be located in thescribe line region 106 between dies 104. In some implementations, theclosed conductive loop 102 is located in the scribe line region 106, butdoes not encircle a die 104. Alternatively, a closed conductive loop 102can be located within a die. In this case, electrical connections to anycircuitry 110 to be used by the integrated circuit but located withinthe loop 102 would need to be routed through conductive lines that passover or under the loop 102 in another conductive layer.

As shown in FIG. 4A, a single wafer 100 typically is fabricated withmultiple dies 104. In some implementations, each die 104 has anassociated conductive loop 102. For example, each die 104 can surroundedby its own conductive loop, or a conductive loop can be located withineach die 104, or a conductive loop can be positioned adjacent each die104 in the scribe line region. Each die can have multiple conductiveloops, and the conductive loops can have the same size or be differentsizes. Eventually the wafer is diced to separate the individual dies.

Although FIGS. 4A and 4B illustrate the loop as generally rectangular,this is not required; the loop could be an arbitrary simple (i.e.,non-self-intersecting) shape, such as an n-sided simple polygon. Theloop can also have one or more curved segments.

The signal strength from the inductive monitoring system 40 will dependon the size of the conductive loop 102 relative to the sensor 42, and inparticular relative to the horizontal dimensions of the prongs 50 andthe distance of the loop 102 from the core 44. The power dissipationthrough a conductive loop is determined by both the magnetic fluxthrough the loop and the electrical resistance of the loop. On the onehand, the smaller the conductive loop, the less magnetic flux will passthrough the loop, and the weaker the signal will be. On the other hand,if the conductive loop is too large, then the magnetic field linesemerging from one of the poles will curve back to the other pole whilestaying within the area of the loop, such that the total magnetic fluxthrough the loop again is reduced. In addition, the electricalresistance of the loop increases linearly with the total length of theloop. This results in a decrease in power dissipation, thus weakersignal, for a sensor with a certain size. In general, the size of theloop should generally match the size of one of the prongs 50 of the core46. For example, the lateral dimension L of the conductive loop 102should be approximately 1-2 times the lateral dimension of one of theprongs 50 of the core 46.

Referring to FIG. 5, the closed conductive loop 102 is fabricatedsimultaneously with the other conductive features in the conductivelayer being fabricated. In particular, a trench is formed, e.g., byetching, in a dielectric layer 112 that has been deposited on a wafer110. The dielectric layer 112 can be a stack of layers, e.g., a low-klayer, a capping layer, etc. A thin barrier layer 114, can be depositedto coat the inside of the trench and the top surface of the dielectriclayer 112. Then a conductive material 116 can be deposited to fill thetrench; the conductive material also covers the top surface of thedielectric layer 112. The conductive material can be a metal, e.g.,copper or cobalt. The barrier layer can be titanium, titanium nitride,or tantalum nitride.

The conductive material 116 is then polished away to expose the topsurface of the dielectric layer 114. It is at this point that thesubstrate reaches the state shown in FIG. 4A. Polishing of the substrate100 can continue until the conductive material 116 in the trench reachesa target depth. During this portion of the polishing step, the depth ofthe trench can be monitored using the inductive monitoring system.Polishing to reduce the trench depth can be performed at the same platenthat is used for exposing the top surface of the dielectric layer 114.

Since the conductive loop 102 is fabricated in the same process as theother conductive components in the layer, the trench of the conductiveloop 102 should have the same depth as the trenches in the die that willprovide the circuitry of the integrated circuit. Thus, monitoring of thethickness of the conductive loop 102 can reasonably be relied upon formonitoring of the thickness of the other conductive features.

In many substrates, there are multiple layers with metal features formedon a substrate. These layers sometimes referred to as M1, M2, etc., withM1 being the metal layer closest to the semiconductor wafer. Referringto FIG. 6, when a substrate with multiple layers is being polished, aconductive loop can be formed in each layer. For example, conductiveloops 102 a, 102 b, 102 c can be formed in metal layers M1, M2, M3,respectively. In some implementations, conductive loops in two differentlayers are substantially aligned, e.g., the conductive loop 102 b isdirectly above the conductive loop 102 a.

A potential problem is that the conductive loops in lower layerscontribute to the measured signal, and consequently serve as a source ofnoise during monitoring of the trench depth in the outermost layer. Insome implementations, the farther the layer is from the substrate, thewider the conductive loop is. For example, the conductive loop 102 c inM3 can be wider than the conductive loop 102 b in M2, which can be widerthan the conductive loop 102 a in M1. In contrast, the lines thatprovide the conductive interconnects of the integrated circuit can havethe same width in each layer.

Due to the increased width of the loop, the loop has a lower resistance.As a result, the signal strength from the loop in each layer becomesconsecutively stronger. For example, the signal strength from theconductive loop 102 c can larger than the signal strength from theconductive loop 102 b, which can be stronger than the signal strengthfrom the conductive loop 102 a. Since the signal strength increases witheach layer, noise resulting from the conductive loops in the lowerlayers has less impact on the signal to noise ratio and the reliabilityof monitoring of the trench depth in the outermost layer.

Optionally, each conductive loop can be electrically connected to theconductive loop in the next lower layer. For example, conductive loop102 c can be electrically connected to conductive loop 102 b, andconductive loop 102 b can be electrically connected to conductive loop102 a.

Referring to FIG. 7, in some implementations, the single conductive loop102 is replaced by a multi-loop structure 122. The structure 122 hasmultiple openings 126 separated by conductive lines 128. The openings126 can be uniformly spaced apart. In some implementations, thestructure 122 is formed by inserting dielectric slits in a metal ringline. The composite structure of the multi-loop structure 122 can bedesigned to have close or similar CMP behaviors as those of criticaldevice trenches or interconnect lines that will form part of theintegrated circuit in the die 102.

The ratio of the area covered by the openings 126 relative to the areacovered by the lines 128 can be selected to match the pattern density ofthe device pattern in the adjacent die. For example, if the devicepattern in the adjacent die has a 50% pattern density, the ratio of thearea covered by the lines to the total area can be 0.5. This permits themetal lines to have a similar response to the CMP operation as thepattern in the die.

Returning to FIGS. 1-3, as noted above, when the magnetic field 50passes through a conductive loop on the substrate, the magnetic field 50generates a current in the loop, which results in a change in signalstrength from the inductive monitoring system. However, since the sensor42 is moving relative to the substrate, and the loops are distributedacross the substrate, the sensor 42 at some times will be located over aregion without a loop, and might only intermittently sweep across a loop10. As a result, the signal from the inductive monitoring system willonly intermittently register the effects from the loops.

FIG. 8 illustrates a graph of a sample signal 130 from a single sweep ofthe sensor 42 across a substrate 100. In the graph, the horizontal axisrepresents distance from the center of the substrate, and the verticalaxis represents the signal strength (in arbitrary units). The signal 130includes an initial portion 132 of low signal strength. The portion 132can represent a time where the sensor is not below the carrier head, sothere is nothing to generate a signal. This is followed by a portion 134of moderate signal strength. This portion 134 can represent a time wherethe sensor is below the retaining ring, so metal parts in the carrier orretaining ring might generate some signal.

There then follows a portion 136 that appears to have significant“noise”, with many individual spikes 140 separated by valleys 142. Ingeneral, over the portion 136, the signal strength does not fall below aminimum 144. Without being limited to any particular theory, the spikes140 can represent times when the sensor 42 is located below a loop, andthe valleys 142 can represent times when the sensor is located below aregion that does not have a loop.

Since the signal strength of the spikes 140 represents the depth of thetrenches, the signal needs to be processed to remove extraneousbackground signal and noise. The signal processing can be performed bythe controller 90.

In general, a signal window is selected. The signal window can representa portion of time that the sensor is scanning across the substrate, or aradial zone on the substrate. Optionally, the signal can initially besubject to a high-pass filter to remove DC portion of the signal whichis not generated by the conductive loops on the substrate. The signalstrength when the sensor is not below the carrier head is measured togenerate a reference value. This reference value is subtracted from thesignal measured while the sensor is below the carrier head, e.g., duringthe signal window. This can compensate for signal drift within apolishing operation for a substrate, e.g., due to chemical or thermalenvironmental changes.

In one implementation, the strength of the signal 130 is averaged overthe entire signal window to generate an average signal value. Theaverage value can be used as the output value. This technique can beappropriate where the conductive loops are uniformly and denselydistributed across the substrate.

In another implementation, individual peaks 140 within the signal windoware identified. The maximum signal strength of each peak 140 isdetermined. The signal strength of the floor, e.g., the average value ofthe valley region between peaks, is subtracted from the signal strengthof each peak to generate a set of peak-to-floor signal values. The setof peak-to-floor signal values from the signal window can be averaged togenerate an average peak-to-floor signal value. The averagepeak-to-floor signal value can be used as the output value. Thistechnique can be appropriate for signals with sparse peaks and a flatfloor, e.g., where the conductive loops are distributed with arelatively low density and are located within each die.

In another implementation, individual peaks 140 within the signal windoware identified. The maximum signal strength of each peak 140 isdetermined. The signal strength of the peaks within the signal windowcan be averaged to generate an average peak value. The average peaksignal value can be used as the output value. This technique can beappropriate for signals with sparse and uneven peaks, e.g., where thereare conductive loops of different sizes within each die and distributedwith relatively low density.

In each of the above implementations, since there is one output valuefor the signal window per sweep, as polishing progresses this generatesa sequence of values which can be used for endpoint detection or closedloop polishing rate control.

It should be understood that the “peaks” can be upward spikes from alower baseline signal, or downward spikes from a higher baseline signal.

FIG. 9 is an example graph of output values 150 generated by theinductive monitoring system during polishing of a device substrate 100.In the graph, the horizontal axis represents time and the vertical axisrepresents the output value. In some implementations, the output valuecan be converted, e.g., using a look-up table, a thickness value, whichprovide the values 150.

In some implementations, endpoint can be called when the current valueof the second spectral feature reaches a target value 152. The targetvalue 152 represents the output of the inductive monitoring system whenthe trench has a target depth.

In some implementations, a function 154 is fit to the output values 152,e.g., using a robust line fit. The function 154 can be used to determinethe polishing endpoint time. In some implementations, the function is alinear function of time. In some implementations, the time at which thefunction 154 equals the target value 152, provides the endpoint time156.

FIG. 10 is an example graph of output values for two different zones onthe substrate 100. For example, the inductive monitoring system 40 cantrack a first zone located toward an edge portion of the substrate 100and a second zone located toward a center of the substrate 100. Asequence of first output values 160 can be measured from the first zoneof the substrate 100, and a sequence of second output values 162 cansimilarly be measured from the second zone of the substrate 100.

A first function 162, e.g., a first line, can be fit to the sequence offirst output values 164, and a second function 166, e.g., a second line,can be fit to the sequence of second values 162. The first function 164and the second function 166 can be used to determine an adjustment tothe polishing rate of the substrate 10.

During polishing, an estimated endpoint calculation based on a targetvalue 168 is made at time TC with the first function for the first zoneof the substrate 100 and with the second function for the second zone ofthe substrate 100. The target value 168 represents the output of theinductive monitoring system when the trench has a target depth. If theestimated endpoint times T1 and T2 for the first and the second zonesdiffer (or if the values of the first function and second function at anestimated endpoint time 170 differ), the polishing rate of at least oneof the zones can be adjusted so that the first zone and second zone havecloser to the same endpoint time than without such an adjustment. Forexample, if the first zone will reach the target value 168 before thesecond zone, the polishing rate of the second zone can be increased(shown by line 172) such that the second zone will reach the targetvalue 168 at substantially the same time as the first zone. In someimplementations, the polishing rates of both the first portion and thesecond portion of the substrate are adjusted so that endpoint is reachedat both portions simultaneously. Alternatively, the polishing rate ofonly the first portion or the second portion can be adjusted.

The sequence of output values provides an output signal. In someimplementations, the output signal can be filtered prior to fitting thefunction. For example, in some situations, the output signal exhibits aregular periodic oscillation. Without being limited to any particulartheory, this might be due to shifting orientation of the substrate fromrotation to rotation of the platen. To compensate for this periodicoscillation, the following algorithm can be applied to the sequence ofoutput values:Processed Signal=sqrt[signal(t)*signal(t)+signal(t−Δt)*signal(t−Δt)]where Δt is one quarter of the period of oscillation. The period ofoscillation can be determined, e.g., by performing a Fourier transformof the output signal and determining the peak frequency strength.

Initially, before conducting polishing, the current generator 62 can betuned to the resonant frequency of the LC circuit, without any substratepresent. This resonant frequency results in the maximum amplitude of theoutput signal.

As shown in FIG. 11A, for a polishing operation, the substrate 100 isplaced in contact with the polishing pad 30. The substrate 100 has theconductive layer 116 covering the underlying patterned dielectric layer112. Since, prior to polishing, the bulk of the conductive layer 112 isinitially relatively thick and continuous, it has a low resistivity. Asa result, the magnetic field from an inductive monitoring system 40 cangenerate eddy currents in the conductive layer. The eddy currents causethe metal layer to function as an impedance source; this permitsmonitoring of the thickness of the substrate during bulk polishing ofthe conductive layer.

Referring to FIG. 11B, as the substrate 100 is polished the bulk portionof the conductive layer 116 is thinned. As the conductive layer 116thins, its sheet resistivity increases, and the eddy currents in themetal layer become dampened. In some implementations, the substrate canbe moved to a different platen when the inductive monitoring system or adifferent monitoring system determines that a predetermined thickness Tof the conductive layer remains over the underlying layers.

Referring to FIG. 11C, eventually the bulk portion of the conductivelayer 116 is removed, exposing the barrier layer 114 and leavingconductive material 116 in the trenches between the patterned dielectriclayer 112 to provide the interconnects 104 a of device and the loopconductor 102. In some implementations, the substrate can be moved to adifferent platen when the inductive monitoring system or a differentmonitoring system, e.g., an optical monitoring system, determines thatthe barrier layer has been exposed.

Referring to FIG. 11D, polishing continues to remove the barrier layer114, exposing the top surface of the patterned dielectric layer 112. Thedepth of the conductive material 116 in the trenches is also reduced. Insome implementations, the substrate can be moved to a different platenwhen the inductive monitoring system or a different monitoring system,e.g., an optical monitoring system, determines that the barrier layerhas been exposed.

If the substrate is subject to both bulk polishing of the conductivelayer and thinning of the dielectric layer at the same platen, thenafter exposure of either the barrier layer 114 or the top surface of thedielectric layer 112, the mode of the inductive monitoring system 40 isswitched from a bulk thickness monitoring mode to trench depthmonitoring mode. In general, in the trench depth monitoring mode, thepeaks in the signal resulting from the conductive loops need to bedetected and extracted from the overall signal to generate the sequenceof values. In contrast, in the bulk thickness monitoring mode, no suchpeaks are expected or extracted, and the raw signal can be averaged tomonitor bulk conductive layer thickness.

Referring to FIG. 11E, with the inductive monitoring system 40 in thetrench depth monitoring mode, the substrate is polished. This thins boththe dielectric layer 112 and reduces the depth of the conductiveinterconnects 116′ in the trenches. As discussed above, the signal fromthe inductive monitoring system 40 can be used to detect a polishingendpoint and halt polishing when the trenches reach a target depth Dand/or modify the polishing rate of different portions of the substrateto improve polishing uniformity.

In some implementations, rather than use the inductive monitoring systemto monitor bulk polishing, the polishing station includes a separateeddy current monitoring system. In some implementations, the polishingstation includes an optical monitoring system. The optical monitoringsystem can be used to detect exposure of the barrier or patterneddielectric layer. Detection of exposure of the barrier or patterneddielectric layer can be used to trigger monitoring with the inductivemonitoring system, or to trigger the inductive monitoring system toswitch from the bulk thickness monitoring mode to the trench depthmonitoring mode.

In some implementations, after the polishing, the substrate is subjectedto a buffing step.

The inductive monitoring systems can be used in a variety of polishingsystems. Either the polishing pad, or the carrier head, or both can moveto provide relative motion between the polishing surface and thesubstrate. The polishing pad can be a circular (or some other shape) padsecured to the platen, a tape extending between supply and take-uprollers, or a continuous belt. The polishing pad can be affixed on aplaten, incrementally advanced over a platen between polishingoperations, or driven continuously over the platen during polishing. Thepad can be secured to the platen during polishing, or there can be afluid bearing between the platen and polishing pad during polishing. Thepolishing pad can be a standard (e.g., polyurethane with or withoutfillers) rough pad, a soft pad, or a fixed-abrasive pad.

In addition, although the foregoing description focuses on monitoringduring polishing, it would also be possible to apply these techniques toan in-line monitoring system. For example, a stationary sensor could bepositioned in a section of the polishing apparatus before the polishingstation, e.g., in the factory interface or in a module attached to thefactor interface. The robot responsible for transporting the substratecould move the substrate past the sensor. Alternatively, the substratecould be positioned on a stand in the factory interface or in a moduleattached to the factory interface, and an actuator could move the sensoracross the substrate while the substrate sits stationary. In eithercase, the series of measurements taken across the substrate can beequivalent to a single scan of the sensor of the in-situ monitoringsystem across the substrate, and can be processed as described above togenerate a measurement of the depth of the trenches.

Embodiments of the invention and all of the functional operationsdescribed in this specification can be implemented in digital electroniccircuitry, or in computer software, firmware, or hardware, including thestructural means disclosed in this specification and structuralequivalents thereof, or in combinations of them. Embodiments of theinvention can be implemented as one or more computer program products,i.e., one or more computer programs tangibly embodied in an informationcarrier, e.g., in a non-transitory machine-readable storage medium or ina propagated signal, for execution by, or to control the operation of,data processing apparatus, e.g., a programmable processor, a computer,or multiple processors or computers. A computer program (also known as aprogram, software, software application, or code) can be written in anyform of programming language, including compiled or interpretedlanguages, and it can be deployed in any form, including as astand-alone program or as a module, component, subroutine, or other unitsuitable for use in a computing environment. A computer program does notnecessarily correspond to a file. A program can be stored in a portionof a file that holds other programs or data, in a single file dedicatedto the program in question, or in multiple coordinated files (e.g.,files that store one or more modules, sub-programs, or portions ofcode). A computer program can be deployed to be executed on one computeror on multiple computers at one site or distributed across multiplesites and interconnected by a communication network.

The processes and logic flows described in this specification can beperformed by one or more programmable processors executing one or morecomputer programs to perform functions by operating on input data andgenerating output. The processes and logic flows can also be performedby, and apparatus can also be implemented as, special purpose logiccircuitry, e.g., an FPGA (field programmable gate array) or an ASIC(application-specific integrated circuit).

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.Accordingly, other embodiments are within the scope of the followingclaims.

What is claimed is:
 1. A method of chemical mechanical polishing asubstrate, comprising: placing the substrate in contact with a polishingsurface and generating relative motion between the substrate andpolishing surface; during polishing of a layer, sweeping a sensor of anin-situ inductive monitoring system across the substrate to generate asignal, the sensor generating a magnetic field that at leastintermittently impinges the substrate; extracting from the signal asequence of values over time, the sequence of values representing adepth of conductive material over time, the extracting includingdetermining a base signal strength for a time period when the magneticfield impinges the substrate, identifying one or more peaks in thesignal for the time period and determining a signal strength for eachpeak, and calculating a value from the sequence of values based on thesignal strength of a peak; and at least one of detecting a polishingendpoint by determining from the sequence of values that the depth ofthe conductive material has reached a target depth, or adjusting atleast one pressure applied by a carrier head to the substrate duringpolishing of the layer based on the sequence of values such thatdifferent zones on the substrate have closer to the same endpoint timethan without such an adjustment.
 2. The method of claim 1, whereinextracting the sequence of values comprises subtracting the base signalstrength from signal strengths of the one or more peaks to generate asignal difference for each peak, and calculating the value from thesequence of values based on the signal difference.
 3. The method ofclaim 2, wherein extracting the sequence of values comprises averagingsignal differences for a plurality of differences from the time period,and calculating the value from the sequence of values based on theaverage of the signal difference.
 4. The method of claim 1, whereinextracting the sequence of values comprises averaging signal strengthsof a plurality of peaks from the time period, and calculating the valuefrom the sequence of values based on the average of the signalstrengths.
 5. The method of claim 1, comprising detecting the polishingendpoint.
 6. The method of claim 5, wherein detecting the polishingendpoint comprising fitting a function to the sequence of values anddetermining a time that the function equals a target value.
 7. Themethod of claim 1, comprising filtering the sequence of values to removea regular periodic oscillation.
 8. A computer program product encoded ona non-transitory computer storage medium, operable to cause a processorto perform operations to control a polishing operation, the operationscomprising: receiving a signal from an inductive monitoring systemgenerated by at least intermittently impinging a magnetic field on asubstrate undergoing polishing of a layer; and extracting from thesignal a sequence of values over time, the sequence of valuesrepresenting a depth of conductive material on the substrate over time,the extracting including determining a base signal strength for a timeperiod when the magnetic field impinges the substrate, identifying oneor more peaks in the signal for the time period and determining a signalstrength for each peak, and calculating a value from the sequence ofvalues based on the signal strength of a peak; and at least one ofdetecting a polishing endpoint by determining from the sequence ofvalues that the depth of the conductive material has reached a targetdepth, or adjusting at least one pressure applied by a carrier head tothe substrate during polishing of the layer based on the sequence ofvalues such that different zones on the substrate have closer to thesame endpoint time than without such an adjustment.
 9. The computerprogram product of claim 8, wherein extracting the sequence of valuescomprises subtracting the base signal strength from signal strengths ofthe one or more peaks to generate a signal difference for each peak, andcalculating the value from the sequence of values based on the signaldifference.
 10. The computer program product of claim 9, whereinextracting the sequence of values comprises averaging signal differencesfor a plurality of differences from the time period, and calculating thevalue from the sequence of values based on the average of the signaldifference.
 11. The computer program product of claim 8, whereinextracting the sequence of values comprises averaging signal strengthsof a plurality of peaks from the time period, and calculating the valuefrom the sequence of values based on the average of the signalstrengths.
 12. An apparatus for chemical mechanical polishing,comprising: a platen having a surface to support a polishing pad; acarrier head to hold a substrate such that a layer on the substratecontacts the polishing pad; an inductive sensor to monitor a conductivematerial on the substrate by generating a magnetic field that at leastintermittently impinges substrate; and a controller configured toreceive a signal from the inductive sensor and extract a sequence ofvalues over time from the signal, the sequence of values representing adepth of the conductive material, the controller configured to extractthe sequence of values by determining a base signal strength for a timeperiod when the magnetic field impinges the substrate, identifying oneor more peaks in the signal for the time period and determining a signalstrength for each peak, and calculating a value from the sequence ofvalues based on the signal strength of a peak, the controller configuredto at least one of detect a polishing endpoint by determining from thesequence of values that the depth of the conductive material has reacheda target depth, or adjust at least one pressure applied by the carrierhead to the substrate during polishing of the layer based on thesequence of values such that different zones on the substrate havecloser to the same endpoint time than without such an adjustment. 13.The apparatus of claim 12, wherein the controller is configured toextract the sequence of values comprises by subtracting the base signalstrength from signal strengths of the one or more peaks to generate asignal difference for each peak, and calculating the value from thesequence of values based on the signal difference.
 14. The apparatus ofclaim 13, wherein the controller is configured to extract the sequenceof values by averaging signal differences for a plurality of differencesfrom the time period, and calculating the value from the sequence ofvalues based on the average of the signal difference.
 15. The apparatusof claim 12, wherein the controller is configured to extract thesequence of values by averaging signal strengths of a plurality of peaksfrom the time period, and calculating the value from the sequence ofvalues based on the average of the signal strengths.